1. Field of the Invention
The present invention relates to a semiconductor device having a circuit formed using a thin film transistor (hereinafter referred to as TFT) and a manufacturing method thereof. For example, the present invention relates to an electro-optic device typified by a liquid crystal display panel, a light-emitting display device having an organic light-emitting element, a wireless chip on which an antenna is mounted, or an electronic device on which a light receiving element or the like is mounted as a component.
Note that in this specification, a semiconductor device refers to all devices that can function by utilizing semiconductor characteristics. An electro-optical device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. In future, thin film transistors are expected to be widely applied to electronic devices such as ICs or electro-optical devices. Thin film transistors having minuter structures need to be developed in accordance with demands for miniaturization and higher performance of semiconductor devices.
In order to manufacture thin film transistors having minute structures, not only conductive films such as gate electrodes, source wirings, or drain wirings but also insulating films typified by gate insulating films are desired to be thinned.
In the case where an integrated circuit into which a thin film transistor is incorporated is mounted on an electronic device, for example, a drive voltage higher than or equal to 5 V and lower than or equal to 10 V is used in a pixel portion of a liquid crystal panel, depending on the mode of liquid crystals. Thus, the thin film transistor provided in the pixel portion preferably has a structure suitable for operation at a drive voltage higher than or equal to 5 V and lower than or equal to 10 V.
Further, as a method for reducing power consumption of liquid crystal display devices, a method for lowering drive voltage can be used. Gate insulating films of thin film transistors are thinned so that the drive capability of the thin film transistors is prevented from decreasing even though the thin film transistors are operated at low voltage.
As a method for forming a gate insulating film having small thickness, a method for forming an oxide film by thermal oxidation can be used. However, since thermal oxidation is high-temperature heat treatment, heat temperature at the time of thermal oxidation exceeds the melting point of a glass substrate in the case of using the glass substrate as a substrate. Further, the thermal oxidation oxidizes part of a semiconductor layer, which leads to decrease in thickness of the semiconductor layer. In the case where the semiconductor layer is extremely thin, there is a possibility that the semiconductor layer disappear.
When a gate insulating film of a thin film transistor is thinned, it is possible to lower voltage which is necessary for switching; however, there is a possibility that the withstand voltage of the thin film transistor be lowered.
Therefore, in order to ensure the reliability of the thin film transistor, the gate insulating film is required to have large thickness to some extent. In a conventional electronic device on which an integrated circuit is mounted, reliability is given top priority. Thus, a gate insulating film having certain large thickness is used.
Not only liquid crystal display devices but also light-emitting display devices having organic light-emitting elements, wireless chips on which antennas are mounted, and light receiving elements have similar problems in that the gate insulating film is required to have large thickness to some extent in order to ensure reliability.
Reference 1 (Japanese Published Patent Application No. 2006-332634) discloses a method for forming a dense insulating film by performing plasma treatment using high frequency on a semiconductor film or an insulating film formed over a substrate having an insulating surface, such as a glass substrate.
Further, Reference 2 (Japanese Published Patent Application No. H11-121763) discloses a capacitor with an insulating film used as a dielectric, which includes a capacitor line and a semiconductor thin film which is the same semiconductor thin film as a channel formation region of a thin film transistor connected to a pixel electrode in order to increase the integration level of a circuit in an active matrix display device.